A pilot receives information from many sources during take-off, flight, and landing of an aircraft. The aircraft includes avionic systems designed to collect data, perform calculations on the data, and present the data to the pilot. For example, the aircraft may include an inertial navigation system (INS), an Attitude Heading Reference System (AHRS), an air data computer, a roll-pitch-yaw computer, a mission computer, various displays, and other avionic systems. Some avionic systems may include one or more sensors that collect data, such as attitude, heading, altitude, and air speed. The same or other avionic systems may process this data. Avionic displays may present the data to the pilot in a usable format.
If one of the sensors becomes inoperable, the pilot may have to rely on the information he can obtain from other sensors to continue the flight and land the aircraft safely. For example, the INS and the AHRS systems may provide similar information to the pilot. Both systems may provide attitude and heading information to the pilot. If there was a problem with the INS, the pilot may obtain some of the same information from the AHRS. Additionally, it is important that the data that the pilot is receiving is accurate. So if the INS is providing data, but the data is erroneous, the pilot should use the data obtained from the AHRS and ignore the INS data.
Typically, when one of the sensors fails or provides erroneous data, the sensor and/or the associated avionic system is deactivated. For example, if the INS fails, the INS would be deactivated and the pilot would rely on data obtained from the AHRS. The pilot would not be able to use data from the INS again unless the pilot manually re-started the INS. Additionally, the pilot has no way of knowing if the INS has resumed providing reliable data until after re-starting the INS. Accordingly, a pilot will typically land the aircraft using the AHRS data and trouble shoot the INS once the aircraft is grounded.
While the previous example is presented using the INS and the AHRS, other avionic systems and/or sensors may provide an overlap of information such that if one fails, the pilot still has access to some data. This redundancy of information provides for safer flights.
Additionally, the redundancy of information may improve the accuracy of some avionic systems. For example, the aircraft may include both an INS and a global positioning satellite (GPS) receiver, or other radio frequency (RF) ranging system, such as Time Difference of Arrival (TDOA) and Galileo. Both the INS and the GPS receiver may provide estimates of the aircraft's position. In addition, the data from the GPS receiver may be used to calibrate the INS, while the GPS receiver may use the data from the INS to quickly re-establish tracking of a satellite in which the GPS receiver has temporarily lost contact. Thus, the integration of the INS and GPS receiver provides more accurate and robust data to the pilot.
The integration of the INS and the GPS receiver may be described as loosely, closely, tightly, or deeply coupled. A loosely coupled system may be described as a stand-alone GPS receiver integrated with a stand-alone INS. The GPS receiver passes position, velocity, and time (PVT) information obtained from four satellites to the INS. The INS uses the PVT information to correct inertial errors that are commonly associated with INS operation. However, the GPS data passed to the INS becomes unusable when less than four satellites are available to the GPS receiver.
A closely coupled system may be described as a stand-alone GPS receiver integrated with a stand-alone INS. However in a closely coupled system, in addition to the GPS receiver passing PVT information obtained from typically four satellites to the INS, the INS passes velocity, acceleration, and angular rate information to the GPS receiver. The GPS receiver can use this information when tracking satellites and to re-acquire a satellite signal that has been lost. However, as with the loosely coupled system, the GPS data passed to the INS becomes less usable when less than four satellites are available to the GPS receiver. For example, when only three satellites are available, GPS receivers typically continue to output horizontal position, but do not output valid altitude data.
In a tightly coupled system, the GPS receiver provides pseudorange and/or deltarange data to the INS. The GPS receiver contains tracking loops for tracking data from multiple satellites. The tracking loops provide pseudorange and deltarange measurements to the INS. The pseudorange measurements are an output of a delay lock loop, which is used for tracking code phase, while the deltarange measurements are an output of a phase lock loop, which is used for tracking carrier phase. The pseudorange and deltarange measurements are used by a Kalman filter in the INS to calculate errors, which sends correction data to a navigation computation.
In the tightly coupled system, the GPS receiver sends pseudorange and deltarange to the INS for all satellites that are being tracked. The INS may continue to use the data obtained from the GPS receiver even when fewer than four satellite signals are being tracked. The INS in a tightly coupled GPS/INS system can continue to use the GPS data with less than four available satellites because each pseudorange and deltarange measurement is an independent measurement.
A deeply coupled system includes a GPS function and an Inertial Measurement Unit (IMU). The GPS function may be defined as the processing associated with computing the GPS data, while the IMU is generally described as the inertial sensing component of the INS, providing data directly to a computer. In a deeply coupled system, a stand-alone GPS receiver may not exist. For example, the functions of the GPS receiver may be resident in a single processor, along with the INS function.
The computer performs the INS computations. However, in contrast with the tightly coupled system which uses pseudorange and deltarange data, measurements from all available satellites are processed by the Kalman filter using in-phase (I) and quadrature (Q) signals, which are calculated in the GPS function. The Kalman filter calculates the errors and sends correction data to the navigation computation and to the GPS function. The information sent to the GPS function includes commands to replica code generators to enable the GPS function to track the GPS satellites. This capability eliminates the need for standalone tracking loops in the GPS function. By combining information from multiple satellites and the inertial sensors, the deeply coupled system is able to track the satellites under higher interference or jamming levels.
Accordingly, the more integrated the GPS/INS system becomes, the more robust the navigation system becomes. Additional benefits may be obtained by integrating data from other sensors. For example, data from the deeply integrated GPS/INS system may be used to calibrate the air data computer and a magnetometer. The air data computer and magnetometer may then be used as aids should GPS data become unavailable and the performance of the INS has degraded to a level where air data or magnetometer aiding will improve the accuracy of the navigation solution.
It would be beneficial to use a deeply integrated GPS/INS system in a navigation system that is operable to automatically resume using data from a sensor that resumes providing reliable data. Accordingly, the pilot may operate the aircraft using the best data available from the avionic sensors.